In general, photovoltaic devices are classified into a monocrystal type, a polycrystalline type and a substantially amorphous type depending on the type of semiconductor layers used as photoelectric conversion layers. The present specification relates to devices having a substantially amorphous conversion layer. In recent years, active studies and developments have been made particularly in connection with photovoltaic devices of the polycrystal type and the substantially amorphous type.
The reason for the attention paid to the just mentioned devices is seen in that it is easy to form such a polycrystalline type or substantially amorphous type photovoltaic device with a large area as compared to a conventional monocrystalline type photovoltaic device. Further, the polycrystalline type or substantially amorphous type photovoltaic device requires a lower energy in its manufacture, and hence a reduction in manufacturing cost can be expected. However, although numerous results of the studies have been obtained, the polycrystalline type or substantially amorphous type photovoltaic device is still inferior in its performance relative to the monocrystalline type photovoltaic device.
Thus, as new attempts to develop photovoltaic devices, studies have been in progress recently for achieving still higher photoelectric conversion efficiencies by forming semiconductor junctions which appropriately combine a substantially amorphous semiconductor layer and a polycrystalline semiconductor layer, or a substantially amorphous semiconductor layer and a monocrystalline semiconductor layer while maintaining their respective preferable properties.
Normally, however, excellent semiconductor junctions cannot be formed merely by contacting semiconductor layers of different conductivity types with each other. For example, even if a monocrystalline semiconductor layer of one conductivity type and a substantially amorphous semiconductor layer of the opposite conductivity type, or a polycrystalline semiconductor layer of one conductivity type and an amorphous semiconductor layer of the opposite conductivity type are directly contacting each other to form a pn junction, a sufficient photoelectric conversion efficiency cannot be obtained, because many of the carriers generated in the semiconductor layers by irradiation with light, disappear by recombination in the vicinity of the interface of the pn junction, and hence the carriers cannot be collected efficiently.
It is considered that the recombination of the carriers is due to localized energy levels in a polycrystalline semiconductor or in an amorphous semiconductor.
That is, generally the quality of a substantially amorphous semiconductor film deteriorates substantially when doped with conductivity type determining impurities. The deterioration is represented as an increase in localized levels within the energy band gap. The localized energy levels increase the energy levels at the interface of the pn junction whereby an increase in the recombination of carriers is caused by the increased interface energy levels.
Further, since a polycrystalline semiconductor film includes a large number of grain boundaries therein and the crystal grains have random orientations, the polycrystalline semiconductor film has increased interface energy levels. Also, there is another disadvantage because impurities are liable to segregate into the grain boundaries in the polycrystalline semiconductor film.